A fundamental aim of internal combustion engines is to minimize fuel consumption while increasing the overall engine efficiency. However, operating methods within a spark-ignition or applied-ignition engine render fuel consumption and efficiency problematic. For example, a conventional spark-ignition engine with intake manifold injection, also referred to as port fuel injection, operates with a homogeneous fuel/air mixture that is prepared by an external mixture formation by introducing the fuel into the air within the air intake manifold. Furthermore, load control is accomplished by means of a throttle valve provided within the intake manifold. In particular, closing of the throttle valve increases a pressure loss induced in the air across the throttle valve, which produces a lower induced air pressure downstream of the throttle valve and ahead of a cylinder inlet. In this way, the air mass (e.g., mass quantity) supplied to the engine cylinder may be adjusted by way of the induced air pressure. This method of load control, however, also has disadvantages, especially in the part load range, wherein low loads may require a high degree of throttling. However, the high degree of throttling may occur via a pressure reduction in the intake section, which results in exhaust and refill losses that rise with a decreasing loads.
In order to lower the above-described losses, various strategies for dethrottling an applied-ignition internal combustion engine have been developed. For example, one approach to dethrottling a spark-ignition engine is to inject fuel directly to the cylinders in the spark-ignition operating method. Thereby, direct injection of the fuel presents a suitable means for achieving a stratified combustion chamber charge, or a stratified charge operation, that allows substantial dilution of the mixture. This allows thermodynamic advantages to be realized, especially in part-load operations (e.g., in the lower and medium load ranges) when small quantities of fuel are injected. For this reason, the methods described herein, which form the subject matter of the present disclosure, employ a direct injection of the fuel into the engine cylinders.
Further advantages may be obtained on the basis of internal cooling, associated with direct injection, of the combustion chamber or of the mixture, thereby making possible higher compression and/or pressure charging and consequently enhanced fuel utilization without premature self-ignition of the fuel, which is referred to as engine knock or knocking, and which is otherwise a characteristic of spark-ignition engines.
A stratified-charge operation is distinguished by a very inhomogeneous combustion chamber charge with an ignitable fuel/air mixture having a comparatively high fuel concentration (e.g., λ<1) formed in the region of the ignition device, whereas a lower fuel concentration, e.g., higher local air ratios (λ>1), is/are present in the mixture layers situated therebelow. Overall, this leads to a lean combustion chamber charge having an overall air ratio λ>>1. In the context of the present disclosure, the air ratio is defined as the ratio of the air mass actually supplied to at least one cylinder of the internal combustion engine to the stoichiometric air mass, or the mass which would be just enough to fully oxidize the fuel mass supplied to the at least one cylinder (e.g., stoichiometric operation of the engine has λ=1).
With regard to direct injection, the fuel/air mixture is likely inhomogeneous during ignition and combustion, especially in a stratified charge operation since the mixture cannot be characterized by a single air ratio, but instead contains both lean mixture components (λ>1) and rich mixture components (λ<1). In particular, the formation of soot that is a characteristic of diesel-type methods is formed in mixture components having a substoichiometric air ratio (e.g., λ<0.7) and/or at temperatures above 1300 K under conditions of extreme oxygen deficiency.
Further, the time available for injecting fuel, preparing the mixture in the combustion chamber, namely the intermingling of air and fuel to a sufficiently desired extent, and preparing the fuel in the context of preliminary reactions, including vaporization, and ignition of the prepared mixture is comparatively short, and may be, for example, on the order of milliseconds. Therefore, in order to ensure reliable ignition of the fuel/air mixture when starting the internal combustion engine, especially during a cold start, previous methods describe injecting a multiple of the fuel mass which may burn stoichiometrically with the charge air in the cylinder during the starting phase. As such, enrichment factors (x) of 10 and above are not uncommon, wherein the enrichment factor x indicates (e.g., defines), the ratio of the fuel mass actually supplied to the stoichiometric fuel mass. By supplying an excess of fuel, the aim of these measures is to vaporize a sufficiently large fuel quantity to ensure reliable ignition. However, a disadvantage is that the excessive amount of fuel also leads to very high raw particulate emissions during the starting phase.
For this reason, to minimize the emission of soot particles, methods are known that employ regenerative particulate filters to filter soot particles out of the exhaust gas for storage until the soot particles are burned intermittently as part of a filter regeneration. For this purpose, oxygen or excess air is included in the exhaust gas to oxidize the soot collected within the filter, which is achieved for example via superstoichiometric operation (λ>1) of the engine.
With regards to filter regeneration, methods are known wherein the filter is regenerated on a regular basis, e.g., at specified fixed intervals. For instance, filter regeneration may be performed based upon reaching a predetermined mileage or time in service. Alternatively, it is also possible for the actual soot loading of the filter to be estimated by means of mathematical models or by measuring an exhaust gas backpressure that arises due to increasing flow resistance of the filter based upon the increased mass of particulates in the filter. Thereby, filter regeneration may be carried out when a maximum permissible loading, which may be specified, is reached. When no catalytic assistance is available, the high temperatures for regeneration of the particulate filter (e.g., about 550° C.) are achieved at high loads and high engine speeds during operation. Therefore, filter regenerations may occur infrequently when the engine is operated for short periods of time.
Frequent cold starts by the engine and/or short journey lengths/durations may further lead to high raw particulate emissions. Thereby, frequent regeneration of the particulate filter may become necessary, however, at the same time, the basic boundary conditions for regeneration of the particulate filter, in particular high temperatures, are not achieved. For this reason, engines are known that are fitted not only with a particulate filter but also with additional exhaust gas aftertreatment systems to reduce pollutant emissions. As such, the particulate filter can be designed in combination with one or more of said exhaust gas aftertreatment systems.
In particular, catalytic reactors are often used with spark-ignition engines. For example, in the case of three-way catalytic converters, nitrogen oxides NOx are reduced by means of the unoxidized components of the exhaust gas that are present, namely carbon monoxides CO and unburned hydrocarbons HC, while, at the same time, these exhaust gas components are oxidized. However, stoichiometric operation (with λ≈1) within narrow limits is necessary for this purpose. In the case of internal combustion engines operated with excess air, e.g., direct-injection spark-ignition engines or lean-burn spark-ignition engines, reducing the nitrogen oxides NOx in the exhaust gas is not possible, owing to the principle involved, that is to say owing to the absence of a reducing agent. Consequently, an exhaust gas aftertreatment system must be provided for the reduction of nitrogen oxides (e.g., a storage-type catalytic converter or a selective catalytic converter).
The inventors have recognized issues with the above-described approaches, and herein describe methods for reducing raw particulate emissions from a direct injection applied-ignition internal combustion engine. In particular, the methods comprise adjusting at least one of a fuel release pressure threshold and enrichment factor based on one or more engine conditions; activating a starting device to rotate a crankshaft coupled to an engine cylinder without injecting any fuel; supplying fuel to the cylinder based on the enrichment factor only when a fuel pressure exceeds the fuel release pressure threshold; and stratifying a cylinder charge while adjusting at least one fuel injection within a compression phase and/or expansion phase of the engine. In this way, the methods ensure that the fuel injected, which may be substantially reduced in some cases, evaporates in the combustion chamber while also preventing a combustion wall wetting due to the high levels of fuel overfueling, which leads to high particulate emissions. Therefore, in view of what has been stated above, one object of the present disclosure is to provide a means for overcoming the known disadvantages and, in particular, for reducing the raw particulate emissions during the starting phase of the engine, which is also regulated to maintain a start duration below a predetermined time threshold.
In one particular example, methods for reducing raw particulate emissions from an applied-ignition engine are described, wherein the engine comprises: at least one cylinder, in which a piston connected to a crankshaft oscillates between a bottom dead center position (BDC) and a top dead center position (TDC) when the internal combustion engine is in operation and in which an injection nozzle is provided for direct injection of fuel; a fuel supply system for supplying the at least one cylinder with fuel; and a starting device, by means of which the crankshaft is forced to rotate during starting. Further, during the starting of the engine, the example methods include activating the starting device in order to impart rotation to the crankshaft, wherein the at least one cylinder may be supplied with fuel only when the fuel pressure (pfuel), in the fuel supply system has reached a threshold pressure, or minimum pressure (PTHRESHOLD) where pfuel≧PTHRESHOLD; and wherein a stratified cylinder charge is produced in the cylinder by means of at least one adjusted injection, for which purpose said at least one injection, in which the majority of the fuel is supplied, is carried out during the compression phase and/or expansion phase of the engine drive cycle.
In the methods according to the present disclosure, fuel is not necessarily injected in the first compression phase of the at least one cylinder or during the first revolution of the crankshaft but is instead injected only when the fuel pressure pfuel in the fuel supply system has reached a minimum pressure PTHRESHOLD. Thereby, the methods further relate to starting an engine from rest; only injecting fuel to a rotating engine after fuel pressure reaches a threshold; adjusting an air-fuel ratio produced by the injected fuel in the engine, the air-fuel ratio enleaned as the threshold is reduced; and spark-igniting the injected fuel in a stratified mixture. Likewise, the method also comprises enriching the air-fuel ratio as the threshold is increased. Assuming equal fuel quantities, a high fuel pressure shortens the duration of injection and further assists mixture preparation in the combustion chamber, in particular the atomization and vaporization of the fuel may occur in an advantageous manner. In this way, the technical result is achieved that allows a high injection pressure, and further makes it possible to introduce at least the majority of the fuel into the cylinder within a small crank angle window, in particular close to TDC. Further, a greater or lesser proportion of the injected fuel may reach the inner wall of the cylinder to mix with the adhering oil film, depending on the quantity of injected fuel and the duration of injection, or injection time. Therefore, it is not only that a portion of fuel may enter the crank case together with the oil and blow by gas for contribution to oil dilution, but that the fuel on the combustion chamber walls, which are cold during starting, contributes greatly to increased raw particulate emissions. Through modification of the lubricating properties of the oil, oil dilution has a substantial influence on wear and durability, e.g., the service life of the internal combustion engine. Thereby, the inventors herein realize that a late introduction of fuel close to TDC presents a suitable measure for substantially minimizing the proportion of fuel that reaches the inner wall of the cylinder during injection, and hence also presents a suitable measure for reducing raw particulate emissions during the starting phase.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.